Mac Maharaj confirmed that the vehicle had engine trouble and that the former president was transferred to another ambulance.

But he said there was no threat to Mr Mandela as he was surrounded by intensive care nurses the whole time.

American network CBS quotes sources as saying he had to wait for 40 minutes.

The CBS report says the transfer to another ambulance took place in freezing winter temperatures.

Mr Mandela, 94, was being transported from Johannesburg to hospital in Pretoria in the early hours of 8 June.

He was admitted in a serious condition with a recurrence of long-standing lung problems and has been in intensive care since. It is his third stay in hospital this year.

Analysis

By Mike WooldridgeBBC News, Johannesburg

The embarrassment for the South African government of the breakdown of the military ambulance taking Nelson Mandela to hospital is obvious - how much it could have affected his condition during the emergency is likely to be argued over intensely here now that the story has emerged.

The presidency - controlling the information about Mr Mandela's health and hospital admissions of this kind - insists that his condition was not "compromised" and says the fact that intensive care nurses were with him throughout should be further reassurance.

But already many here want to know what the word "serious" in the government's statements about Mr Mandela's health really means before they will take much encouragement from the talk of improvement.

The need to respect Mr Mandela's dignity is widely recognised. But the government knows public trust is at stake too.

There has been little information about his condition for some days. President Jacob Zuma said on 13 June that his health continued to improve but his condition remained serious.

More recently, one of Mr Mandela's grandsons, Ndaba Mandela, said his grandfather was getting better and he hoped he would be home soon.

Convoy

Mr Maharaj confirmed the ambulance breakdown in an interview with local TV station, ENCA.

"I appreciate the concern caused by this," he said.

"I want to assure the public that from the presidency side we are assured by the doctors that all care was taken to ensure that former President Nelson Mandela's medical condition was not compromised by this incident."

Mr Maharaj said Mr Mandela was in a convoy with a full complement of medical staff and no-one could have predicted the engine problem. "It happens in life," he said.

The presidential spokesman dismissed speculation surrounding Mr Mandela's medical condition, calling for things to be done "in a dignified way" and urging the media to rely on updates from the presidential office.

After leading the struggle against white minority rule under the apartheid system, Mr Mandela became South Africa's first black president in 1994.

He was jailed for 27 years for his role in the fight against apartheid and is believed to have suffered damage to his lungs while working in a prison quarry.

He contracted tuberculosis in the 1980s while being held in prison on the windswept Robben Island.